Digital circuits, because of their better stability, higher speed, lower cost, smaller size, lower power consumption, and reduced sensitivity to the surrounding environment, are now performing many of the functions previously performed by analog circuits. However, many systems, particularly those involving data communications devices linked together by a central office telephone line, require that control and data signals be converted into an analog form for transmission.
Many of these systems use modems (data modulator-demodulator) for the digital-to-analog and analog-to-digital conversion. Several different digital modulation schemes have therefore been developed for use in modems. See, for example, U.S. Pat. No. 4,049,909, issued to Peck on Sept. 20, 1977, and U.S. Pat. No. 4.504,802, issued to Heatherington on Mar. 12, 1985. The Peck U.S. Pat. No. 4,094,909 discloses a quadrature differential phase shift keyed (QDPSK) digital modulator using a frequency divider, a pulse stuffing counter, a three stage shift register, and a weighted combiner comprising three weighting resistors and an analog summer. Although the digital modulator disclosed by Peck is useful it has certain disadvantages and limitations.
First, since an analog summer is used, the output voltage is sensitive to changes in the resistance of the weighting resistors and to changes in the output resistance of the individual stages in the shift register. Of course, precision, temperature stable weighting resistors and matched, temperature stable shift register stages can be used but this increases the cost of manufacture.
Second, the possible output voltages are determined by the weighting resistors and the feedback resistors used in the analog summer. Changing the peak-to-peak output voltage and the individual quantized output voltage steps therefore requires different resistance values to be selectably inserted. This also raises the cost of manufacture since more precision, temperature stable resistors are required and the devices which switch the resistors must also have a stable output resistance.
Third, if two independent signals are to be added, such as the high tone and low tone to form the dual-tone, multiple-frequency singal used for dialing, two sets of resistors and switching devices are required, again increasing the cost of manufacture.
Fourth, under some data input conditions, where the desired differential phase shift is 180.degree. or 270.degree. and the analog output should be its mean value, 0.5.times.(V.sub.MAX +V.sub.MIN), the actual analog output will be offset from this mean by as much as .+-.0.086.times.(V.sub.MAX -V.sub.MIN), thereby introducing distortion and transient DC signals into the QDPSK signal.
Fifth, the pulse stuffing counter is not well suited for generating phase shifted signals other than QDPSK signals.
As noted above, many digital modulators are species of digital-to-analog converters. However, most digital-to-analog conversion methods suffer from one or more of the following limitations: the relatively high cost of conventional, N-bit digital-to-analog converters, the need for precision bit-weighting resistors, temperature sensitivity, undesirable power consumption, undesirable size, complex control schemes, limitation to use for a particular type of analog signal, use of more than one of a relatively high cost device, etc.
There is, therefore, a need for a digital modulator, particularly for use in modems, which does not suffer from the above-mentioned limitations.